The accomplishment of attaining high-temperature superconductivity is of immense scientific and technological importance. Several critical transition temperature barriers have been breached since the long-standing record temperature of 23.2 Kelvin for Nb.sub.3 Ge was exceeded. However, the ultimate benchmark of room temperature superconductivity remained an unattainable goal until the instant inventors successfully introduced atoms of a parametric modifier into a class of ceramic defect oxide type superconducting materials (described in detail hereinafter) so as to form a new, high critical temperature class of superconducting materials.
In late 1986, the superconducting properties of certain ceramic defect oxide type materials, which materials are variations of a well-known class of inorganic structures called perovskites, were observed by Bednorz and Mueller. The Bednorz and Mueller work was based upon materials developed by Michel and Raveau. The materials which Bednorz and Mueller observed contained lanthanum, barium, copper, and oxygen, and were reported to be superconducting at a temperature of about 30 degrees Kelvin.
Subsequently, many workers in the field became involved in efforts that resulted in the increase of the critical temperature, T.sub.c, (the temperature at which electrons are able to move through a material without encountering any resistance to that motion) by the substitution of yttrium for lanthanum. Upon analysis, the superconducting composition was found to be a perovskite defect oxide of the Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.y type, possibly an orthorhombically distorted perovskite. This composition has also been referred to as a tetragonal, square-planar structure with the Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.y defect oxide perovskite phase being responsible for the superconducting properties. Further work with this high T.sub.c phase effectively raised the critical temperature thereof to a few degrees above 90 degrees Kelvin (a temperature above the atmospheric boiling point of liquid nitrogen).
However, up to this point in time, scientists in the field had been unable to raise the temperature of superconducting materials to the temperature of the environment, i.e., room temperature. More particularly, prior to the work of the instant inventors at the laboratory at Energy Conversion Devices, Inc., three plateaus in raising the critical temperature of superconducting materials had been broached. The first plateau was represented by the pre-ceramic defect oxide type superconducting materials which materials were limited to a T.sub.c of about 23-25 Kelvin; the second plateau was represented by the work of Bednorz and Mueller on lanthanum, barium, copper and oxygen systems, which systems resulted in superconducting temperatures of about 30 degrees Kelvin; and the third plateau was represented by the yttrium, barium, copper and oxygen systems, which systems provided for a rise in critical temperature to about 90-95 degrees Kelvin. However, it was not until the aforementioned new class of parametrically modified, fluorinated, superconducting materials were developed by Energy Conversion Devices, Inc. that the crucial benchmark of room temperature superconducting appeared to be attainable.
Later workers in the field have attempted the total and/or partial replacement of the yttrium and/or lanthanum with other Group IIIA metals (including Rare Earth elements), especially with scandium, europium, lutetium, neodymium, praseodymium, and gadolinium. The same and other workers in the field have also attempted the total and/or partial replacement of barium with other group IIA metals, as strontium and calcium.
The defect oxide perovskite phase, having the general composition M.sub.1.sup.IIIIA M.sub.2.sup.IIA M.sub.3.sup.IB O.sub.y, was identified by several groups utilizing electron microprobe analysis, x-ray diffraction, scanning electron microscopy, neutron scanning and transmission electron microscopy. These groups have independentally characterized this defect oxide perovskite, M.sub.1.sup.IIIIA M.sub.2.sup.IIIA M.sub.3.sup.IB O.sub.y A phase as having the crystallographic unit cell structure shown in FIG. 1.
The perovskite structure is similar to the naturally occurring calcium titanate structure, CaTiO.sub.3, also shown by other AB.sub.2 O .sub.3 -type oxides having at least one cation much larger then the other cation or cations, including the tungsten bronzes, NaWO.sub.3, strontium titanate, barium titanate, YAlO .sub.3, LaGaO.sub.3, NaNbO.sub.3, KNbO.sub.3, KMgF.sub.3, KNiF.sub.3, and KZnF.sub.3, among others. In the perovskite structure the larger ions (La.sup.+3 =1.15 angstroms, Ba.sup.+2 =r 1.35 angstroms, and 0.sup.+2 =1.40 angstroms, Linus Pauling, The Nature of the Chemical Bond, 3rd Edition, Table 13-3, "Crystal Radii and Univalent Radii of Ions") form a cubic close packed structure, with the smaller ions (Cu.sup.+2 =0.96 angstroms, Y.sup.+3 =0.90 angstroms, Pauling, op. cit.) arranged in occupying octahedral interstices in an ordered pattern. Together they form a cubic close packed (face centered cubic) array.
The superconducting perovskite type materials are ceramic defect oxides. That is, the superconducting phases of the perovskite type materials are solids in which different kinds of atoms occupy structurally equivalent sites; where, in order to preserve electrical neutrality, some sites are unoccupied or vacant. Since these vacancies can be filled with mobile oxygen atoms, local order is prevalant. It is also clear that these vacant atoms form lattice defects, which defects have, generally, profound affects on the electrical parameters of the material and, more particularly profound affects on the oxidation states of the copper atoms.
In order to achieve yet higher critical temperatures, researchers at Energy Conversion Devices, Inc. realized that it would be necessary to develop a truly electronic material in which the chemistry thereof was engineered so as to alter the local chemical and electrical environment. For example, and as mentioned hereinabove, it has been established that the mobility of oxygen atoms in the Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7 -type materials is very high and therefore the location of those mobile oxygen vacancies at any point in time contribute to the variations in the local electronic environment. It is this oxygen mobility and changing local environment which results in the unstable electronic properties of this class of superconducting materials. Therefore, one task (the importance of which cannot be overemphasized) established by the instant inventors and emphasized herein was the stabilization of oxygen mobility in high transition temperature phases of the superconducting material.
The structure shown in FIG. 1 has a recurring structure of (1) a M.sup.IB -0 plane of a first type with vacant 0 sites, (2) a M.sup.IIA -0 plane, (3) a N.sup.IB -0 plane of a second type with fully occupied 0 sites, (4) a M.sup.IIIA plane with 0 sites, (5), another M.sup.IB -0 plane of the first type with fully occupied 0 sites, (6) another plane of the M.sup.IIA -0 type, and a (7) a second M.sup.IB -0 plane of the first type, with 0 site vacancies. It may thus be seen that the unit cell (of the superconducting material) so formed has seven planes spacedly disposed along the c axis thereof.
The central plane is a plane of the M.sup.IIIA -0 type, as a Y-0 or La-0 plane, with the Group IIIA metal being surrounded at its four coplanar corners by oxygen sites, which may be occupied or vacant. Immediately above and below this M.sup.IIIA -0 plane are equivalent M.sup.IB -0 planes of the second type, i.e., Cu-0 planes, with the Group IB metal ions being at the four corners of the plane, and occupied oxygen sites being along each edge of the planes. These square planar M.sup.IB atoms (or ions), each surrounded by four oxygen atoms (or ions) have been reported to be critical to superconductivity in the defect oxide perovskites. A pair of M.sup.IIA -0 planes, as Ba-0 planes lie atop and below these fully occupied first type M.sup.IB -0 planes. The M.sup.IIA -0 planes, formed with the Group IIA metal, as barium, at the center have fully occupied oxygen sites, with the oxygens disposed directly above and below the Group IB metal sites of the adjacent planes. The M.sup.IIA -0 planes are disposed between M.sup.IB -0 planes, as shown in FIG. 1, with the first type M.sup.IB -0 planes disposed on opposite sides thereof relative to the second type M.sup.IB -0 planes. As mentioned above, the deficiencies, that is, the vacancies (unoccupied sites) reported to reside in the first type M.sup.IB -0 planes are the result of the requirement of electrical neutrality. While the vacancies are generally reported to be in the M.sup.IB -0 planes, they may also be in the other planes, as in the M.sup.IIA -0 planes and/or in the M.sup.IIIAl - 0 planes.
While the aforementioned researchers have been responsible for the developments which occurred over the course of the last several months in raising the critical temperature of the aforementioned classes of superconducting materials up to about 90 degrees Kelvin; it was not until the work of the instant inventors that the critical temperature of the high T.sub.c phases of said superconducting material has been raised beyond about the 90-98 degree Kelvin plateau. More particularly, the instant inventors have previously provided evidence (in the commonly assigned application referred to the following paragraph) of superconducting phases with transition temperature as high as 155 to 168 degrees Kelvin at which temperatures a zero resistance state has been measured through the use of "modification" techniques. These materials also gave magnetic indication of the presence of yet higher temperature superconducting phases of said superconducting material. In addition, electrical conducting measurements reveal the "modified" superconducting materials of the instant inventors achieved a resistance value approximately four times lower than that of single-crystal copper before the zero-resistance state thereof is reached. This is taken as clear evidence for the presence of same volume fraction of other higher transition superconducting phases. It is also essential to note that recent magnetic measurements of the modified superconducting material indicate superconducting phases as high as about 370 degrees Kelvin.
The inventors of the instant invention, while working with the perovskite, ceramic defect oxide class of superconducting materials, have previously been successful in introducing at least one "parametric modifier" into the unit cell thereof so as to improve the superconducting properties of that class of superconducting materials. This introduction of parametric modifier elements through the use of "shake and bake" powder metallurgical process was fully disclosed in commonly assigned, copending U.S. application Ser. No. 043,279, filed Apr. 27, 1987 by Stanford R. Ovshinsky and Rosa Young entitled "Parametrically Modified Superconducting Material", the disclosure of which is incorporated herein by reference. The incorporation of the parametric modifier into the superconducting phases resulted in overcoming what had appeared to be a barrier to raising the critical temperatures of superconducting material to additional heights. It was in this way that Ovshinsky, et al provided a new mechanism for affecting fundamental parameters of said superconducting materials, which parameters determine the critical temperature thereof. It was also in this manner that the door was opened for further increases in critical temperatures of
superconducting materials, even beyond the 155-168 degree Kelvin temperatures reported in said Ovshinsky, et al application. More specifically, the addition of the parametric modifier, fluorine, to the superconducting material resulted in the identification of a superconducting phase of said defect oxide type superconducting material which has recently been shown to achieve a critical temperature at about 90.degree. F. (a temperature well above room temperature) with indications of the presence of yet higher temperature superconducting phases. It should be specifically noted that the significance of room temperature superconductivity is more than a quantative phenomena; said room temperature critical temperature makes possible a host of new applications which were heretofore unattainable.
While the aforementioned introduction of a parametric modifier (such as fluorine) into a superconducting material successfully raised the critical temperature thereof to at least above about 90 degrees F, which "room" temperature superconductivity represented the ultimate goal long sought after by workers in the field; said introduction of the parametric modifier element was accomplished by a mixing and baking powder metallurgical process (see Example I of the instant specification) wherein at least one fluorinated precursor compound was substituted for at least one oxygenated precursor compound. However, the instant inventors discovered that regardless of the nature of the fluorinated precursor compound employed (i.e., whether the introduction was a BaF or aCuF compound), the baking process required such high temperatures so as to effect a solid state reaction which resulted in the preferential formation of new non-superconducting phases in the bulk of the superconducting material. This preferential formation of fluorinated non-superconducting phases not only prevented significant atomic percentages of fluorine, the parametric modifier, from being incorporated so as to form the high T.sub.c phases of the superconducting material; but it also preferentially formed new non-superconducting phases. More specifically, the addition of fluorinated precursors in said powder metallurgical process resulted in the formation of a large atomic fraction of a BaF.sub.2 non-superconducting phase due to the solid state reaction which released the barium from the superconducting phase. The barium, having been freed, then acts as a getter of the fluorine, thereby giving rise to said new BaF.sub.2 non-superconducting phase.
It was based upon this work that the instant inventors realized (1) significant atomic fractions of new non-superconducting phases of the fluorinated defect oxide type superconducting materials would be preferentially formed and (2) most of the fluorine would be preferentially incorporated into said new non-superconducting phases if typical powder metallurgical processes continued to be employed. Of course, the more fluorine is incorporated into non-superconducting phases, the less fluorine is available to be incorporated into the high T.sub.c phases. These conclusions were due to the preferential formation of said non-superconducting phases at the elevated temperatures required for the solid state powder metallurgical reactions to occur. It was also based upon the aforementioned rationale that the instant inventors developed an "exchange" technique (which technique will be described in detail hereinafter) so as to provide a mechanism for the controllable substitution of large percentages of the labile atoms of the parametric modifier element for constituent atoms of the unit cell of the high T.sub.c superconducting phases of the superconducting material. Additionally, this means of parametric modifier introduction is capable of being employed in low cost, high product throughput manufacturing processes. It also appeared that such an exchange could be effected without the concurrent formation of new, non-superconducting phases. This mechanism is rendered possible by first forming a conventional superconducting material characterized by a relatively high atomic percentage of superconducting phases (such as Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7) and then exchanging a controllable percentage of labile atoms of a parametric modifier element, such as fluorine, for a controllable percentage of the oxygen atoms thereof. By accomplishing this exchange of fluorine for oxygen at a temperature well below the solid state reaction temperature of the yttrium, barium and copper metals, new non-superconducting phases are substantially and kinetically prevented from being formed.
While specific materials will not be disclosed herein, it must be noted that the basic method of exchanging fluorine and oxygen atoms, disclosed in detail hereinafter, can be readily extrapolated so as to provide for the "modulation" and "doping" of other elements within the instant or other superconducting classes of materials. In other words, while, in its most specific sense, the instant invention relates to the controlled substitution of fluorine atoms for oxygen atoms so as to increase the transition temperature or to add stability or to increase the volume fraction of high T.sub.c phases; other elements (such as Y or Ba) of the superconducting phases could be "exchanged" so as to parametrically modify a superconducting related property of the material.
As used herein, the phrase "parametric modifier" refers to the modification of the local environment and/or the local chemistry of the superconducting material in such a manner as to affect one or more parameters which control or impact upon the level of the critical temperature of the superconducting phase. Examples of such control parameters are those related to the BCS model (described in the aforementioned commonly assigned copending U.S. application Ser. No. 043,729), such as Debye Temperature, number of charge carriers above the Fermi sea, coupling constants and parameters related thereto. The Debye temperature, stated in simplified form, is a function of the stiffness of the lattice; however, it is possible that in a superconductive structure of this complexity, an equivalent structural or other mechanism may be operative in affecting the critical temperature. Hence, the term "effective Debye Temperature" will be used hereinafter to refer to a parameter of this general type.
In summary, the parametric modifier thus acts to modify the local chemistry or local environment of the unit cells and/or other structural and chemical configurations from which the superconducting material is fabricated so as to realize changes in the parameters (particularly parameters relative to the oxidation states of the copper atoms or the local electronic environment) affecting the critical temperature. The parametric modifier may also act to affect the interaction between the otherwise shielded orbitals of adjacent atoms of the unit cell, in particular the d orbitals and in some cases the f orbitals as well. The parametric modifier can additionally act to produce changes in certain parameters which are positive in their effects on the critical temperature while at the same time avoiding otherwise related adverse changes in other parameters which would negatively affect the critical temperature. Thus, normally dependent parameters are uncoupled from one another.
Through yet another mechanism, the parametric modifier may act to modify the local electrical environment of the superconducting material so as to provide control of the oxidation states. In this manner, a particular preferred oxidation state for higher critical temperatures of superconducting material may be locked in and/or stabilized.
The local environments in such materials generally include oxygen vacancies which may be viewed as deviations from periodicity or in local order which affect the total interactive environment (TIE). The ability of the modifier to affect such variations from normal structural bonding (NSB) or such deviant electronic configurations (DECs) through modification of the local environment allows manipulation of the critical parameters which affect critical temperature.
It is to be noted that one of the inventors of the subject invention, Mr. Ovshinsky, has previously discussed the coupling and decoupling of various physical properties of superconducting materials by controlling, on a molecular level, the relationship existing between the atoms thereof; however, while applicable to this class of superconducting material, this coupling and decoupling was accomplished for a different class of superconductors and required precision fabrication techniques. See, for example, U.S. Pat. No. 4,520,039, entitled Compositionally Varied Materials And Method For Synthesizing The Materials; U.S. Pat. application Ser. No. 705,241 of the same title; and U.S. Pat. application Ser. No. 026,596 entitled Superconducting Materials. In comparison thereto, the technique for introducing the special parametric modifiers of the instant invention provides for the coupling and decoupling of physical and electrical properties of superconducting materials of the ceramic defect oxide type without the necessity of resorting to complicated fabrication techniques.
Therefore, the instant invention provides a method of controllably exchanging a given preselected percentage of the labile atoms of said parametric modifier for other constituent atoms of a ceramic defect oxide type superconducting material. Evidence is further provided that this exchange can occur without simultaneously introducing non-superconducting phases thereinto.
More particularly, the subject invention discloses a method of exchanging the labile atoms of said parametric modifier for the oxygen atoms of the ceramic oxide type superconducting material in such a manner that the percentage incorporation of labile atoms increases in proportion to the percentage decrease of oxygen atoms released from said material; and conversely the percentage incorporation of labile atoms decrease in proportion to the percentage increase of fluorine atoms from said material. This labile exchange of fluorine atoms for oxygen atoms can result in increased stability, a higher critical temperature and a higher volume fraction of high T.sub.c superconducting phases. It should be noted that fluorine was selected as the labile exchange atom because of its small size and high electronegativity so as to form stronger bonds which lock in desired oxidation states.
These and other objects and advantages of the subject invention will become apparent from the Detailed Description of the Invention, the Drawings and the Claims which follow.